1. Field of the Invention
The present disclosure relates to a method of fabricating a thermoelectric material and a thermoelectric material fabricated thereby.
2. Description of the Related Art
Thermoelectric materials are energy conversion materials in which electric energy is generated when a temperature difference is applied between two ends of the material, and conversely, the temperature difference between the two ends of the material is generated when electric energy is applied. In the beginning of the 19th century, thermoelectric phenomena, such as Seebeck effect, Peltier effect, and Thomson effect, were discovered, and in the late 1930s, thermoelectric materials having a high thermoelectric figure of merit were developed along with the development of semiconductors. In recent years, thermoelectric materials have been used for special power supplies for a mountainous and remote area, space applications, or military applications using thermoelectric power generation, and precision temperature control in semiconductor laser diodes or infrared detectors, computer-related mini coolers, cooling devices for lasers in fiber-optic communication, cooling devices in hot and cold water dispensers, semiconductor thermostats, or heat exchangers using thermoelectric cooling.
Thermoelectric performance, such as power generation capacity and cooling capacity, of a thermoelectric material may be known through a dimensionless figure of merit, ZT=(σ2α/k)T (where σ is a Seebeck coefficient, α is electrical conductivity, k is thermal conductivity, and T is absolute temperature). A high thermoelectric figure of merit of a thermoelectric material denotes a high energy conversion efficiency of the thermoelectric material, and in order to increase the thermoelectric figure of merit, electrical conductivity and Seebeck coefficient must be improved and thermal conductivity must be decreased. Therefore, a thermoelectric material requires independent control of properties for obtaining high electrical conductivity and low thermal conductivity.
In general, electrical conductivity and thermal conductivity of a material are interdependent with each other. That is, it is known that a material having high electrical conductivity also has high thermal conductivity. However, with respect to a thermoelectric material, an appropriate combination of high electrical conductivity and low thermal conductivity is required as illustrated in the above thermoelectric figure of merit (ZT). Among parameters influencing the thermoelectric figure of merit, since the See beck coefficient and electrical conductivity are mainly dependent on scattering of electric charges and thermal conductivity is mainly dependent on scattering of lattice waves, there is a need to control properties through control of a microstructure in consideration of the above parameters. That is, a decrease in thermal conductivity is induced by decreasing the scattering of electric charges in the thermoelectric material as much as possible and increasing the scattering of phonons constituting the thermoelectric material, and consequently, the thermoelectric figure of merit may be improved.
Recently, research into nanocrystallization of thermoelectric materials and fabricating thermoelectric materials including a nanophase has been actively conducted in order to fabricate thermoelectric materials having a high thermoelectric figure of merit (ZT). That is, the reason for this is that since new interfaces between dispersed phase/thermoelectric phase formed by grain boundaries of the thermoelectric material and the dispersed phase may induce active phonon scattering by introducing the dispersed phase with nanometer scale into a thermoelectric matrix material, thermal conductivity may be effectively reduced, and thus, the thermoelectric figure of merit (ZT) may be effectively improved. Since the wavelength of phonons is in a range of 1 nm to 2 nm and the wavelength of electrons is in a range of 10 nm to 50 nm, thermal conductivity may be effectively decreased while electrical conductivity is maintained, when a nanomaterial having a size of 10 nm or less is used. That is, since the movement of carriers may not be greatly affected when the size of the dispersed phase (nanomaterial) is 10 nm or less, the decrease in electrical conductivity due to the introduction of the dispersed phase may be addressed. Therefore, research for improving a thermoelectric figure of merit by fabricating an inner-type thermoelectric material, in which a nano-dispersed phase exists in grains of the thermoelectric matrix material, has been continued.
“Method of preparing thermoelectric material by mechanical milling-mixing method” was disclosed in Korean Patent No. 10-0795194. The above patent provides a method of preparing a thermoelectric material having controlled thermal/electrical characteristics through mechanical milling of a thermoelectric material into different sizes, and mixing, forming, and sintering processes of the milled thermoelectric material, and the thermoelectric material thus prepared. The patent has advantages in terms of polycrystallization and nanostructurization of a single material. However, according to the above-described invention, since it is difficult to prevent agglomeration of nanoscale fine dispersed phases by mechanical mixing and the dispersed phases exist on surfaces of bismuth telluride metal powder rather than exist in the powder, the patent may not be used for fabricating a thermoelectric material using a nano-dispersed phase that is aimed in the present invention.
“Thermoelectric material and composite material using the same as a raw material, and method of preparing the thermoelectric and composite materials” was disclosed in Korean Patent Application Laid-Open Publication No. 10-2011-0128432. The above patent describes a method of preparing a thermoelectric material, in which a first solution having carbon nanotubes dispersed therein and a second solution having a metal salt mixed therein are mixed, a mixed powder generated by a chemical reaction is then mechanically milled, mixed and heat treated to prepare a thermoelectric material having a portion of the carbon nanotubes inserted thereinto, and a thermoelectric composite material is prepared by a spark plasma sintering process of the thermoelectric material. However, the above method has limitations in that the preparation process is complicated because a pre-treatment process of carbon nanotubes and a post-treatment process of milling and mixing the prepared mixed powder are included, electrical conductivity is decreased due to damages in surface structures of the carbon nanotubes, and electrical conductivity of the carbon nanotubes is decreased in the thermoelectric material because the thermoelectric material is prepared by using carbon nanotubes subjected to the pre-treatment process including an acid treatment.
Therefore, research into minimizing processes, such as a milling process and an acid treatment, and directly synthesizing a thermoelectric material powder having carbon nanotubes uniformly dispersed therein from a metal salt has been continuously conducted.
Accordingly, during research into a method able to improve a thermoelectric figure of merit of a thermoelectric material, the present inventors found that when carbon nanotubes with no acid treatment are used, the thermoelectric figure of merit of the thermoelectric material may be improved due to an increase in electrical conductivity and a decrease in thermal conductivity, thereby leading to completion of the present invention.